SPECIAL REPORT/GLASS: A Cool Ride

For decades, glass has been used to protect consumers in
automobiles from external conditions such as rain, wind and bugs. In the last
few decades, developments in glass technology have enabled glass to provide
significantly more value to car companies and consumers. Glass technology has
allowed cars to become quieter, to protect consumers from harmful ultraviolet
(UV) rays, to remain cooler in the summer, and to provide a user interface for
various displays, among other benefits.

The most recent developments in glass technology have been recognized by the
state of California
in its drive to reduce carbon dioxide (CO2) emissions.
California recently approved a regulation that will require energy-efficient
glass in all vehicles sold in California, starting with the 2012 model year. It
is expected that other states will follow suit, and the annual reduction of
national-and, ultimately, global-CO2 emissions could be
in the billions of kilograms.

Efficiency and Comfort

One of the most pressing issues in the world
today is climate change. Scientists around the world have concluded that
greenhouse gas emissions need to be controlled, and the automobile has been
identified as a major contributor of CO2 emissions. In response, the automotive industry
has developed several technologies that reduce CO2 emissions. Some of the most common of these
technologies involve the source of power for the engine.

The
introduction of hybrid electric vehicles (HEVs) was received enthusiastically
by the media and environmentalists. The
interest shown by consumers, as well as the support provided by governments
around the world, has led to the development of plug-in hybrid electric
vehicles (PHEVs) and complete electric vehicles (EVs). All of these
technologies, along with others that help with the propulsion of the vehicle,
have been demonstrated to provide a direct link to fuel consumption and hence
to CO2
emissions.

Traditional
metrics of measuring fuel consumption have centered around the fuel efficiency
associated with propulsion of the vehicle. In reality, several other sources of
fuel consumption exist that are not directly related to the movement of the
vehicle.

The vehicle also acts as a sanctuary for
consumers while they are in it. The consumer expects to be safe, secure and
comfortable while in the vehicle. Vehicle manufacturers have gone to
great lengths to ensure that consumers are as comfortable in the vehicle as
they would be in a home or office environment.

The climate control systems in vehicles, for example,
can be far superior to those found in most homes in the U.S. When a
consumer gets into a car that has been sitting in the sun for a while, the
vehicle’s air conditioning system is so powerful that it cools the extremely
hot interior down to a comfortable temperature in minutes. The large air
conditioning systems required for this level of cooling consume a lot of
energy. Highly sophisticated glass technology can be used to reduce this energy
consumption.

Figure 1. Normalized solar power distribution.

Background

Solar energy consists of three segments (see
Figure 1). The UV spectrum makes up only about 5% of the total energy, but it
has been shown to cause skin cancer with extensive exposure and can also cause
damage to interior materials used in vehicles. Comprising about 43% of solar
energy, visible light is what enables us to see and is thus regulated for
vision glass applications. The remaining 52% of solar energy comes in the
infrared (IR) spectrum, which can therefore be a significant source of heat in vehicles
if it is allowed to enter the vehicle.

Glass technology has long been used to reduce the heat load in automobiles. The
use of solar-absorbing glass has provided significant improvements over the
clear glass that had been in use for many decades. Solar-absorbing glass soaks
up the solar energy and then re-radiates it. In a moving vehicle, most of the
absorbed energy is convected away by the wind. However, in a parked or
slow-moving vehicle, a lot of that energy gets radiated into the vehicle.

Figure 2. By reflecting the IR energy away from the glass, glazing reduces the absorbed energy in the glass and thus reduces the energy radiated into the vehicle.

To address this situation, a glazing technology was developed
several years ago that reflects the IR portion of the solar spectrum while
still allowing the visible light to be transmitted.* By reflecting
the IR energy away from the glass, the glazing reduces the absorbed energy in
the glass and thus reduces the energy radiated into the vehicle (see
Figure 2). The glazing reflects over 400% more energy from the surface than
conventional absorbing glass.

Sputtering technology is used to deposit nano-layers of several materials
to achieve the IR reflective performance. The glass produced using this
technology can reject more than 50% of the solar energy while a vehicle is
parked.

Testing conducted in conjunction with the
U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL)
has shown that use of the glazing in a vehicle can reduce internal breath air
soak temperatures by as much as 5°F. This reduction in interior soak
temperature leads to an 11% reduction in the air conditioning compressor load,
and thus a 2.2% improvement in fuel economy over the U.S. Environmental
Protection Agency (EPA) test cycle that includes the use of air conditioning.

The reduction in soak temperature also means that the consumer is more
comfortable when first entering the vehicle, and the vehicle is able to reach
an acceptable temperature sooner once the air conditioning is turned on.
Testing by the NREL showed that the time to comfort can be as much as 30%
faster.

Figure
3. The new glazing was shown to reduce the breath air soak temperature by as
much as 12°F.

A New Solution

Recently, a new automotive glass glazing was
developed that takes this performance even further.** The technology
rejects almost 97% of IR energy while still maintaining the 70% visible light
transmission required for automotive vision glazing.

The solar heat load reduction of the new
glazing was tested in a vehicle as part of a project led by the Society of
Automotive Engineers. The NREL-run study was designed to look at the impact of
heat load reduction on the air conditioning workload and the fuel economy of
the vehicle. The new glazing was shown to reduce the breath air soak
temperature by as much as 12°F (see Figure 3). This head load reduction would
reduce the air conditioning workload by as much as 25%. It has been shown by
the NREL and the EPA that a 30% heat load reduction can save the U.S. 2.5
billion gallons of fuel and over 22 billion Kg of CO2 emissions annually.

Figure 4. The glazing configuration vehicle had the
highest impact in the UDDS drive cycle.

As vehicles become more fuel efficient, heat
load reduction technologies become even more important. When the engine is
optimized for the highest levels of fuel efficiency, the relative amount of
energy used for auxiliary loads, such as air conditioning, grows higher. To
test this concept, a study was conducted with the NREL on a Toyota Prius hybrid
vehicle that was modified to be a PHEV. Testing was done with a simulated heat
load reduction using a glazed windshield and rear window. The base case was the
vehicle with standard glass. The two configurations were run through an EPA
Urban Dynamometer Driving Schedule (UDDS, for city driving) and a US06 Drive
Cycle (for highway driving).

The vehicle with the new glazing had the highest impact in the UDDS drive
cycle, with the adjusted, utility-factor weighted fuel economy increasing from
36.8 mpg for standard glass to 42.9 mpg for the glazed glass, a difference of
6.1 mpg (see Figure 4). In addition to this fuel savings, the temperature in
the vehicle was still cooler for the glazed case by 4.5°F after the vehicle had
run through its drive cycles with the air conditioner on for both cases.

Multiple Benefits

Testing
has shown that developments in advanced automotive glazing can help improve
vehicle efficiency while increasing consumer comfort. In addition, solar load
reduction technologies can save consumers money in the form of reduced fuel
expenses while also reducing greenhouse gas emissions and U.S. dependence on
foreign oil in the process.